lib/packet/packet_utils: optimize checksum

builtin function, sum, is much faster than for loop.
The result on my machine is as follows

> def main():
>     from timeit import timeit
>     data = bytearray().zfill(1500)
>     print 'new=', timeit(lambda : checksum(data), number=1000)
>     print 'old=', timeit(lambda : checksum_old(data), number=1000)
>
> new= 0.00800108909607
> old= 0.266770124435

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
ryu/lib/packet/packet.py: should use isinstance instead of __class__.__base__

The current implementation doesn't allow inheriting twice from class PacketBase.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>